Mobile communication devices are in widespread use throughout metropolitan regions of the world. These devices are increasingly common and affordable, and to remain competitive manufactures have sought to include additional functionality in them. For example, manufactures are now including positioning receivers in mobile communication devices to support a number of location applications, such as location reporting for emergency services, and navigation.
The use of satellite positioning receivers in mobile communication devices was initially driven by safety concerns related to locating a person calling emergency services using a mobile communication device. Although it is simple to determine the cell location of a mobile caller, the area encompassed by a cell may be quite large, especially in rural areas. Therefore some governments are now requiring that mobile communication devices include a means for determining their own location and reporting it when necessary, such as when the mobile communication device user calls an emergency phone number. Although numerous methods of approximate location determination have been developed, using such techniques as triangulation and relative power levels of signals received from base station radios in the vicinity of the mobile communication device, these methods have not proven sufficiently reliable or precise.
The preferred means of providing location determination in a mobile communication device is to include a satellite positioning receiver. However, this approach is not without significant design challenges. For one, the time needed for a satellite position receiver to lock onto a sufficient number of positioning satellite signals from an autonomous or “cold” start can be significant, perhaps several minutes or longer, even in good signal conditions. Network aiding can reduce the time to first fix (TTFF) in producing a location report to a few seconds to a minute, depending on signal conditions. Network aiding can include GPS time, approximate location of a base station, and satellite ephemeris.
One reason for this latency in TTFF between aided fixes performed on a network and unaided fixes performed off-network is frequency error in the reference oscillator used in the mobile communication device. The reference oscillator is typically susceptible to error from its nominal assigned frequency induced by temperature, thermal drift, age, and manufacturing tolerances, among other factors. High stability and precise oscillators are available, but they are so expensive that in the realm of mobile communication devices, where price is a critical market factor, it is impractical to use them. An alternative to the high priced compensated oscillator is to rely on the frequency precision of a communication system, where the mobile communication device locks onto a precision frequency signal transmitted by a base station, and determines its own frequency error. Thus, the precision frequency signal allows a satellite positioning receiver equipped mobile communication device to reduce the time to first fix by eliminating the frequency error in performing the fix.
However, some mobile communication devices are being manufactured with the ability to communicate directly with other mobile communication devices. Direct communication allows mobile communication devices to communicate with each other even when outside of communication system coverage. Some mobile communication devices may be designed that only communicate directly with other mobile communication devices, and are not capable of network communication. The precision frequency signal of a network base station is unavailable to mobile communication devices outside the coverage area of a communication system, as well as mobile communication devices that are unable to communicate with the network. Therefore there is a need for a way in which a mobile communication device operating in a remote area outside the coverage of a communication system can reduce the time needed to perform a location determination.